In recent years, fiber-reinforced resin composite materials (hereinafter suitably referred to as “composite materials”) have been widely used in fields in which metal materials have been used so far. Among the composite materials, carbon-fiber-reinforced materials which are formed such that carbon fibers used as reinforced fibers, are impregnated with matrix resin such as epoxy resin, have a smaller weight and a higher strength than metal materials. Because of this, the carbon-fiber-reinforced materials are widely used in fields of sport activity articles, industrial machines, and aerospace.
Among these fields, in the field of aerospace, for example, in structures such as wings or bodies of aircraft, a stiffened panel in which a skin comprising a composite material is integrated with a stiffener which is a lightweight metal frame member, has been used. As a typical stiffener, there are a stringer and a frame. The stringer is a member having a relatively small thickness and is placed to extend in a lengthwise direction of the structure. The frame is a member having a relatively great thickness and is placed to cross the stringer in a direction (direction perpendicular to the lengthwise direction) of the transverse cross-section of the structure. The stringers are supported by jigs and arranged to conform to the shape of the structure. Plural sheets of prepreg are stacked together with the stringers, and the resulting stack is pressurized and heated in an autoclave. As a result, the prepreg is cured into skin, and the stringers are adhesively attached to and integrated with the skin. Further, the frame is attached to the stringers, thereby forming a stiffened panel.
There are various kinds of stringers depending mainly on a difference in cross-sectional shape. To be specific, there are known a flat plate shape, a columnar shape, a C-shape, an I-shape, an L-shape, a Z-shape, and a hat shape. Among these, the hat stringer has recently attracted an attention, because it is suitable for weight saving of a structure, for the reason described below.
The aircraft is required to minimize its weight, but is required to have a sufficient strength. Because of this, a thickness of the skin which is a body of the stiffened panel cannot be set constant, but is set greater in a region required to have a higher strength and smaller in a region which can provide a sufficient strength, thereby achieving a reduced weight. The jig supporting the stringer is an elongated member extending along the stringer, and defines a positional relationship between the prepreg and the stringer. The jig is drawn out along its lengthwise direction after the prepreg is cured. Because of this, if the skin has a uniform thickness, a surface of the skin which contacts the stringer is flat. Therefore, it is relatively easy to draw out the jig having supported the stringer after the prepreg is cured. However, if the skin has many non-uniform thickness regions, unevenness is generated in the surface of the skin which contacts the stringer. The jig is also curved to correspond to the unevenness. In this case, it is difficult to draw out the jig.
The hat stringer has a pair of flat band-shaped portions extending in a lengthwise direction of the stringer, and a channel portion having a transverse cross-section recessed in a trapezoidal shape between the band-shaped portions. If the structure is a body of the aircraft formed of a one piece barrel (OPB), the band-shaped portions are surfaces (adhesion surfaces) adhesively attached to the inner surface of the skin. The hat stringers are placed along the lengthwise direction of the body with the adhesion surfaces being at an outer side, and the frame is placed in a circumferential direction of the body, thereby forming an assembly having a predetermined structure, for example, a cylindrical structure. The prepreg is wound around outside of the cylindrical structure and stacked thereon. Furthermore, a jig such as a mandrel is attached to the interior of the cylindrical structure. In a space formed between the skin and each of the hat stringers, i.e., inner side of the channel portion, an elongated jig called a bladder is inserted.
The bladder is made of a material having flexibility and elasticity (stretchability). The bladder is inflatable (expandable) and contractible and has a structure of an elongated bag-shape. During a state where the bladder is inserted as the jig into the channel portion, an internal pressure is increased to allow the bladder to be inflated (expanded). Thus, the bladder maintains a state where the prepreg is stretched under tension to prevent the prepreg from being deformed between the band-shaped portions. At a time point when the prepreg has been cured into the skin, the internal pressure is lowered, and the bladder is contracted and drawn out of the channel portion. At this time, the band-shaped portions of the stringer are adhesively attached to and integrated with the inner surface of the skin. Therefore, it may be difficult to draw out a normal jig due to unevenness on the inner surface of the skin. However, at this time, the bladder has changed its state from the inflated state to the contacted state to have a smaller cross-section and has a flexibility, and therefore, the bladder can be easily drawn out of the channel portion without being affected by the unevenness.
The bladder has a function similar to that of an inflatable mandrel. An exemplary inflatable mandrel has a configuration in which the mandrel is composed of plural layers and has a cylindrical bag shape inflatable and contractible, as disclosed in Patent Literature 1. In this configuration, during molding, the mandrel is inflated and serves as an inner mold (core) with respect to an outer mold. In the contracted state, the mandrel is easily inserted into a cavity of the outer mold, and is easily drawn out of a molded drum-shaped object. As a specific manufacturing technique using the inflatable mandrel, for example, Patent Literature 2 discloses a manufacturing technique of a racket using the inflatable mandrel, and Patent Literature 3 discloses a technique for manufacturing a spar of a rotary wing aircraft using the inflatable mandrel.